U.S. patent application number 11/371312 was filed with the patent office on 2006-09-14 for humidity adjusting film.
Invention is credited to Takafumi Namba, Haruo Nomi.
Application Number | 20060204833 11/371312 |
Document ID | / |
Family ID | 36287192 |
Filed Date | 2006-09-14 |
United States Patent
Application |
20060204833 |
Kind Code |
A1 |
Nomi; Haruo ; et
al. |
September 14, 2006 |
Humidity adjusting film
Abstract
Humidity adjusting films are sandwiched between the catalyst
electrode layers and carbon fiber collector layers of a solid
polymer type fuel cell. These humidity adjusting films are
constructed from a conductive carbonaceous powder and a
polytetrafluoroethylene. The moisture permeability as measured by
the method stipulated in JIS L 1099 (B-1) is 1200 to 4000 g/m.sup.2
hr, and the mean thickness of the films is 5 to 100 .mu.m.
Inventors: |
Nomi; Haruo; (Akaiwa-shi,
JP) ; Namba; Takafumi; (Okayama-shi, JP) |
Correspondence
Address: |
GORE ENTERPRISE HOLDINGS, INC.
551 PAPER MILL ROAD
P. O. BOX 9206
NEWARK
DE
19714-9206
US
|
Family ID: |
36287192 |
Appl. No.: |
11/371312 |
Filed: |
March 7, 2006 |
Current U.S.
Class: |
429/413 ;
252/511; 429/483; 429/521; 429/530; 429/532; 429/534 |
Current CPC
Class: |
H01M 8/04119 20130101;
Y02E 60/50 20130101; H01M 8/1004 20130101; H01M 4/8605 20130101;
H01M 4/8896 20130101; H01M 8/0234 20130101; H01M 4/8817
20130101 |
Class at
Publication: |
429/044 ;
252/511 |
International
Class: |
H01M 4/94 20060101
H01M004/94; H01B 1/24 20060101 H01B001/24; H01M 4/96 20060101
H01M004/96 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 10, 2005 |
JP |
JP2005-067743 |
Claims
1. A humidity adjusting film for use in a solid polymer type fuel
cell, said humidity adjusting film sandwiched between a catalyst
electrode layer and a gas-permeable carbon fiber collector layer,
wherein said film comprises a conductive carbonaceous powder and
polytetrafluoroethylene, the moisture permeability of said film is
1200 to 4000 g/m.sup.2hr, and the mean thickness of the film is 5
to 100 .mu.m.
2. The humidity adjusting film according to claim 1, wherein the
through-type electrical resistance is 30 m.OMEGA.cm.sup.2 or
less.
3. The humidity adjusting film according to claim 1, wherein the
film is calendered.
4. The humidity adjusting film according to claim 1, wherein the
amount of polytetrafluoroethylene is 5 to 60 mass % relative to the
total amount of conductive carbonaceous powder and
polytetrafluoroethylene.
5. The humidity adjusting film of claim 1 wherein said film and
said catalyst electrode layer are integrated by lamination.
6. The humidity adjusting film according to claim 1 wherein said
film and said gas-permeable carbon fiber collector layer are
integrated by lamination.
7. The humidity adjusting film of claim 6, wherein said carbon
fiber collector layer is subjected to a water-repellency treatment
using a fluororesin.
8. The humidity adjusting film of claim 6, wherein the thickness of
said carbon fiber collector layer is 100 to 500 .mu.m.
9. A gas diffusion electrode, wherein a catalyst electrode layer is
integrated by lamination on a surface of the humidity adjusting
film of the laminated gas diffusion layer according to any of claim
6.
10. A fuel cell membrane electrode assembly constructed from a
polymer electrolyte membrane; and a pair of electrode layers that
sandwich this polymer electrolyte membrane from both sides, wherein
a pair of the humidity adjusting films according to claim 1
laminated so as to sandwich said electrode layers from both outer
sides.
11. A gas diffusion layer integrated type membrane electrode
assembly, wherein gas-permeable carbon fiber collector layers are
laminated on both outer sides of the membrane electrode assembly
according to claim 10.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a solid polymer type fuel
cell (polymer electrolysis fuel cell (PEFC)).
BACKGROUND OF THE INVENTION
[0002] In principle, water is the only reaction product of fuel
cells that utilize hydrogen and oxygen. Accordingly, such fuel
cells have attracted attention as a means of generating clean
energy that places little burden on the environment. In particular,
solid polymer type fuel cells have seen active research and
attempts at practical adaptation, as fuel cells that are easy to
handle and that promise to afford an increase in the output power
density. Such fuel cells have a broad range of applications;
examples include sources of the motive force for mobile bodies such
as automobiles and buses, stationary power supplies for general
household use, power supplies for compact mobile terminals and the
like.
[0003] Solid polymer type fuel cells are constructed by stacking
numerous single cells; each single cell typically has a structure
of the type shown in FIG. 1. Specifically, a polymer electrolyte
membrane (ion exchange membrane) 10 is sandwiched from both sides
by a pair of catalyst electrode layers 20 and 21, and these
catalyst electrode layers 20 and 21 are further sandwiched from
both sides by a pair of carbon fiber collector layers (also called
porous supporting layers or gas diffusion layers) 40 and 41. The
outer sides of these carbon fiber collector layers 40 and 41 are
opened toward gas flow channels (fuel gas flow channels 50 and
oxygen-containing gas flow channels 51) formed by separators 60 and
61. Furthermore, the fuel gas (H.sub.2 or the like) introduced from
the flow channels 50 passes through the first carbon fiber
collector layer (anode side carbon fiber collector layer) 40, so
that protons (H.sup.+) are produced while electrons are released by
the anodic electrode reaction shown below, which takes place at the
first catalyst electrode layer (anode, fuel pole). These protons
next pass through the polymer electrolyte membrane 10, and receive
electrons as a result of the cathodic electrode reaction shown
below, which takes place at the second catalyst electrode layer
(cathode, oxygen pole), so that H.sub.2O is produced.
Anodic electrode reaction: H.sub.2.fwdarw.2H.sup.++2e.sup.-
Cathodic electrode reaction:
1/2O.sub.2+2H.sup.++2e.sup.-.fwdarw.H.sub.2O
[0004] Furthermore, a perfluoro type electrolyte as represented by
Nafion (commercial name, manufactured by Du Pont), or a hydrocarbon
type electrolyte, is commonly used as the abovementioned polymer
electrolyte membrane 10. In order for such polymer electrolyte
membranes 10 to conduct protons, the co-presence of H.sub.2O is
required. Furthermore, the catalyst electrode layers 20 and 21 are
constructed from a catalyst metal and a proton-conducting
electrolyte, and H.sub.2O is also required in order to promote the
electrode reactions in these electrode layers 20 and 21.
Ordinarily, therefore, a supply of water vapor (humidification) is
performed via the gases supplied (fuel gas 50, oxygen-containing
gas 51) in order to maintain the polymer electrolyte membrane 10
and catalyst electrodes layers 20 and 21 of the fuel cell in an
appropriate water-containing state during operation.
[0005] The H.sub.2O that is supplied for humidification to the fuel
gas 50 dissolves in the electrolyte contained in the anode
electrode layer 20 and in the polymer electrolyte membrane 10, and
moves to the cathode side together with the movement of protons. A
portion of the H.sub.2O that is not utilized is discharged to the
outside of the system as water vapor together with the exhaust gas
50, while the remainder of this H.sub.2O is discharged to the
outside of the system as condensed water via a drain (not shown in
the figures). Furthermore, the H.sub.2O that is supplied to the
oxygen-containing gas 51 for humidification is similarly dissolved
in the electrolyte contained in the catalyst electrode layer 21 and
in the polymer electrolyte membrane 10, and the H.sub.2O that is
not utilized is either discharged to the outside of the system
together with the exhaust gas 51, or discharged to the outside of
the system as condensed water via the drain (not shown in the
figures). In addition, a portion of the H.sub.2O that is produced
by the electrode reaction of the cathode catalyst layer 21
undergoes reverse diffusion through the polymer electrolyte
membrane 10, and moves to the anode side where this H.sub.2O is
utilized, while the remainder of this H.sub.2O passes through the
carbon fiber collector layer 41 on the cathode side, and is
discharged to the outside of the system as water vapor or condensed
water.
[0006] As a result of such H.sub.2O supply, H.sub.2O generation and
H.sub.2O utilization, the cathode electrode layer 21 assumes a
relatively H.sub.2O-rich state. It is necessary to maintain this
H.sub.2O at an appropriate level by causing the water vapor
pressure difference or H.sub.2O concentration difference to act as
a driving force on the side of the carbon fiber collector layer 41,
and by causing the H.sub.2O concentration difference to act as a
driving force on the side of the polymer electrolyte membrane
10.
[0007] In the case of mobile bodies such as automobiles and the
like, load fluctuations of the fuel cell frequently occur during
starting, driving and stopping. Accordingly, it is desirable that
it be possible to operate such fuel cells mounted in mobile bodies
under a broad range of operating conditions ranging from low output
to high output. Furthermore. there are severe restrictions in terms
of mounted weight and capacity, so that the fuel cell must be
compact and light-weight. Moreover, it is also necessary to devise
added equipment such as the gas supply device (pump or the like)
and humidifying device so that this added equipment has a small
power consumption and is light in weight. For example, the gas flow
rate from the gas supply device is generally set at approximately
40 to 50% in terms of the air utilization rate. However, if the air
utilization rate can be increased even further, the power
consumption and weight of the gas supply device can be reduced.
Furthermore, in order to realize a reduction in the power
consumption and weight of the humidifying device, it is desirable
that the amount of humidification of the polymer electrolyte
membrane 10 required during the operation of the fuel cell be
minimized (low humidification operation, dry operation).
[0008] However, if the amount of humidification is reduced, the
water vapor pressure difference between the catalyst electrode
layer 21 and carbon fiber collector layer 41 is increased, so that
the amount of H.sub.2O that moves from the polymer electrode
membrane 10 and catalyst electrode layer 21 to the carbon fiber
collector layer 41 increases. As a result, the H.sub.2O content of
the polymer electrolyte membrane 10 is lowered, so that the proton
conductivity is lowered, or the catalyst electrode layer 21 is
dried so that the effective catalyst area is reduced, thus leading
to a so-called dried-up state so that the output of the fuel cell
is decreased, and it may become impossible in some cases to
maintain the generation of electric power.
[0009] For example, even if the operating conditions are not dry
operating conditions, if the amount of power generation is
increased (e.g., if high-output operation at a current density of
approximately 1 Acm.sup.2 or greater is performed), the amount of
accompanying H.sub.2O that moves through the polymer electrolyte
membrane 10 to the side of the catalyst electrode layer 21
increases. Furthermore, the amount of heat generated by the
catalyst electrode layer 21 becomes conspicuous, and the water
vapor pressure difference between the catalyst electrode layer 21
and the carbon fiber collector layer 41 is increased; consequently,
the H.sub.2O in the electrode layer 21 moves to the side of the
carbon fiber collector layer 41 in large quantities, so that a
dried-up state may appear in some cases.
[0010] When the operating state is a dried-up state, the useful
life of the polymer electrolyte membrane 10 is shortened;
consequently, high-humidification conditions are unavoidable. Even
in the case of stationary fuel cells used in household
applications, operation under low-humidification conditions is
desirable from the standpoint of low power consumption; however,
since the useful life of the membrane is shortened,
high-humidification conditions must be employed. As was described
above, however, since the cathode electrode layer 21 intrinsically
tends to assume an H.sub.2O-rich state, if the fuel cell is
operated under high-humidification conditions, a so-called flooding
state tends to result in which the water content of the cathode
electrode layer 21 becomes excessive. In this flooding state, the
electrode layer 21 and carbon fiber collector layer 41 are wetted
with water; as a result, the supply of oxygen-containing gas to the
catalyst metal is blocked, so that the output of the fuel cell is
decreased. Furthermore, in the case of the abovementioned
high-output operation (operation at a current density of 1
Acm.sup.2 or greater), there may also be cases in which dry-up (in
which water is taken from the polymer electrolyte membrane 10) and
a flooding state that occurs because of the insufficient discharge
of H.sub.2O to the side of the carbon fiber collector layer 41 from
the electrode layer 21 both appear at the same time.
[0011] Various techniques have been proposed in order to prevent
dry-up and flooding. For example, the void ratio of the second
carbon electrode maybe gradually increased from the upstream side
of the oxidizing agent flow path toward the downstream side.
Referring to the abovementioned FIG. 1, this means that the void
ratio of the carbon fiber collector layer 41 on the cathode side is
gradually increased moving in the direction of depth from the front
side of the plane of the page. Furthermore, a mixed layer
consisting of a fluororesin and carbon black may be formed between
the catalyst layers 20 and 21 and the carbon fiber collector layers
40 and 41, and the thickness of the mixed layer in the portions 50a
and 51a on the side of the inlets of the fuel gas and
oxygen-containing gas (oxidizing agent gas) is set so that this
thickness is greater than the thickness of the portions 50b and 51b
on the side of the outlets. However, these techniques, since the
void ratio or thickness varies in a gradation in the plane
direction of the carbon fiber collector layers 40 and 41, the
pressure distribution becomes nonuniform when the single cells are
stacked up and tightened, so that the performance is not
stable.
[0012] Furthermore, a carbon layer may be formed by coating on the
surfaces of the carbon fiber collector layers (gas diffusion
substrates) 40 and 41 located on the sides of the catalyst layers
20 and 21, and these carbon layers are separated into island form
or lattice form within the plane of the layers, so that gap parts
are formed between the separated carbon layers. However, since the
carbon fibers generally stand up in the form of a nap on the
surface of a carbon fiber collector layer, numerous indentations
and projections are present. Since the carbon fiber collector
layers are merely coated with carbon layers, the abovementioned nap
or indentations and projections are not reduced, and there is a
danger that the electrode layers 21 or polymer electrolyte membrane
10 will be scratched by the pressure that is applied when the
single cells are stacked up. Furthermore, even if a technique is
devised in which carbon layers formed into sheets beforehand are
laminated with the carbon fiber collector layers in order to
prevent such damage, it is necessary to split these sheets (carbon
layers), and since the gaps that are formed by this splitting are
extremely large, areas of water accumulation tend to be formed very
easily, so that flooding conversely tends to occur.
[0013] Furthermore, there is absolutely no disclosure as to how
both dry-up and flooding can be prevented over a broad range
extending from high-humidification conditions to low-humidification
conditions, and over a broad range extending from a high gas flow
rate (high air utilization rate) to a low gas flow rate.
[0014] Furthermore, sheets obtained by the paste extrusion and
calendering of a powdered PTFE--carbon black mixture (formed into
sheets beforehand) may be integrated by lamination with the carbon
fiber collector layers (carbon papers) 40 and 41. However, in this
technique, there is no description of the prevention of dry-up or
prevention of flooding. Furthermore, the thickness of the
abovementioned sheet-form substance (powdered PTFE--carbon black
mixture) is 0.2 mm (200 .mu.m) or 0.6 mm (600 .mu.m).
SUMMARY OF THE INVENTION
[0015] With the foregoing aspects in view, it is an object of the
present invention to establish a technique which makes it possible
to prevent both dry-up and flooding of a fuel cell under a broad
range of operating conditions ranging from high-humidification
conditions to low-humidification conditions, and ranging from a
high gas flow rate (low air utilization rate) to a low gas flow
rate (high air utilization rate).
[0016] In order to solve the abovementioned problems, the present
inventors first of all conducted a preliminary investigation in
order to ascertain whether it is better to endow the carbon fiber
collector layers (gas diffusion layers) 40 and 41 themselves with a
humidification adjusting function by impregnating these layers 40
and 41 with a water-repellent (or hydrophilic) substance, or
whether it is better to laminate a film with these carbon fiber
collector layers 40 and 41, and to endow this film with a
humidification adjusting function. As a result, it was discovered
that in the case of the impregnation method, water tends to
condense in the large voids inside the carbon fiber collector layer
41, possibly because the carbon fiber collector layer 41, which has
an extremely coarse structure, is directly laminated with the
catalyst electrode layer 21, so that the prevention of flooding is
difficult. Accordingly, it was decided to pursue a further detailed
investigation of the lamination of a film with the carbon fiber
collector layers 40 and 41, and it was ultimately found as a result
of numerous trial and error experiments that the thickness and
moisture permeability of the laminated film are important causative
factors.
[0017] Generally, the relationship between the moisture
permeability of such a laminated film (humidity adjusting film) and
the phenomena of dry-up and flooding is unclear. For example, even
if the moisture permeability of this film is maintained at
approximately 2000 g/m.sup.2 hr, there may be some cases in which
it is possible to generate power without causing dry-up or flooding
(see Working Example 2 and Working Example 4 described later), and
other cases in which dry-up occurs so that power cannot be
generated (see Comparative Example 2 described later).
[0018] Generally, furthermore, the relationship between the
thickness of this laminated film (humidity adjusting film) and the
phenomenon of dry-up or flooding is also unclear. Specifically, if
the thickness of the film is increased, the heat insulation between
the catalyst electrode 21 and carbon fiber collector layer 41 is
increased, so that the temperature difference increases;
accordingly, the water vapor pressure difference also increases.
This acts to accelerate the movement of H.sub.2O from the catalyst
layer 21 to the side of the carbon fiber collector layer 41. On the
other hand, as the thickness of the film increases, the distance
between the catalyst electrode 21 and carbon fiber collector layer
41 increases, so that the H.sub.2O concentration gradient drops.
This acts to decelerate the movement of H.sub.2O from the catalyst
electrode 21 to the side of the carbon fiber collector layer
41.
[0019] Accordingly, when various investigations were conducted
regarding the abovementioned relationship, it was found that both
dry-up and flooding can be prevented under a broad range of
conditions only when both the film thickness and moisture
permeability of the laminated film (humidity adjusting film) are
controlled to specified ranges. The present invention was perfected
as a result of this discovery.
[0020] Specifically, the humidity adjusting film of the present
invention is constructed from a conductive carbonaceous powder and
a polytetrafluoroethylene, the moisture permeability of this film
as measured by the method stipulated in JIS L 1099 (B-1) is 1200 to
4000 g/m.sup.2 hr, and the mean thickness of the film is 5 to 100
.mu.m. Furthermore, the humidity adjusting film of the present
invention is used by being sandwiched between the catalyst
electrode layers and carbon fiber collector layers of a solid
polymer type fuel cell.
[0021] In this humidity adjusting film, the polytetrafluoroethylene
ordinarily constitutes 5 to 60 mass % of the total of the
conductive carbonaceous powder and polytetrafluoroethylene, and the
through-type electrical resistance as measured by the four-terminal
method (1 kHz alternating current, pressure between terminals 981
kPa, room temperature) is ordinarily 30 m.OMEGA.cm.sup.2 or less.
It is desirable that the humidity adjusting film be subjected to a
calendering treatment.
[0022] When used in a single cell, the abovementioned humidity
adjusting film may also be formed into a composite film by
laminating and integrating this film with various types of layers
beforehand. For example, the humidity adjusting film of the present
invention may be laminated and integrated with a catalyst electrode
layer (humidity adjusting film equipped with an electrode
function), the humidity adjusting film of the present invention may
be laminated and integrated with a carbon fiber collector layer
(gas diffusion layer) to form a laminated type gas diffusion layer,
or a catalyst electrode layer may be laminated and integrated with
the humidity adjusting film surface of this laminated type gas
diffusion layer (gas diffusion layer equipped with an electrode
function). Furthermore, the humidity adjusting film of the present
invention may be a film in which a polymer electrolyte membrane is
sandwiched between a pair of catalyst electrode layers from both
sides, and then further sandwiched between a pair of the humidity
adjusting films of the present invention from both sides (membrane
electrode composite body), or may be a film in which gas-permeable
carbon fiber collector layers are laminated with both sides of such
a membrane electrode composite body (gas diffusion layer integrated
type membrane electrode composite body).
[0023] Furthermore, in the respective composite films described
above, the carbon fiber collector layer(s) may be subjected to a
water-repellent treatment by means of a fluororesin. Moreover, the
thickness of the carbon fiber collector layer(s) is (for example)
approximately 100 to 500 .mu.m.
[0024] In the present invention, a mixture of a conductive
carbonaceous powder and a polytetrafluoroethylene is formed into a
film beforehand, the moisture permeability and thickness of this
film are controlled to specified ranges, and this film is
sandwiched between a catalyst electrode layer and a carbon fiber
collector layer (gas diffusion layer). Accordingly, both dry-up and
flooding can be prevented under a broad range of operating
conditions ranging from high-humidification conditions to
low-humidification conditions, and ranging from a high gas flow
rate (low air utilization rate) to a low gas flow rate (high air
utilization rate).
DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a schematic perspective view showing a
conventional fuel cell (single cell).
[0026] FIG. 2 is a schematic perspective view showing one example
of the fuel cell (single cell) of the present invention.
[0027] FIG. 3 is a schematic perspective view showing another
example of the fuel cell (single cell) of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The present invention will be described in greater detail
below with appropriate reference to the attached figures. FIG. 2 is
a schematic perspective view showing one example of a fuel cell
single cell using the humidity adjusting film of the present
invention. Structural parts that are the same as in FIG. 1 are
labeled with the same symbols, and a description of such parts is
omitted.
[0029] In the refractive index, in order to adjust the single cell
with a humidity adjusting function, instead of subjecting the
gas-permeable carbon fiber collector layers (gas diffusion layers)
40 and 41 themselves to any modification (impregnation treatment
such as a water-repellent treatment or hydrophilicizing treatment),
films (humidity adjusting films) 30 and 31 are laminated with the
surfaces of these carbon fiber collector layers 40 and 41 that are
located on the sides of the catalyst electrode layers 20 and 21,
and these films are endowed with a humidity adjusting function. In
cases where an impregnation treatment is performed, water tends to
condense in the large voids inside the carbon fiber collector
layers 40 and 41. On the other hand, in cases where such films 30
and 31 are laminated, this problem can be avoided, and flooding can
easily be prevented. Furthermore, in cases where these films 30 and
31 are laminated, damage to the catalyst electrode layers 20 and 21
by nap or indentations and projections on the surfaces of the
carbon fiber collector layers 40 and 41 can also be prevented.
Furthermore, in the humidity adjusting film of the present
invention, the thickness and void ratio are substantially uniform
in the plane direction; accordingly, the tightening pressure that
is applied when the single cells are stacked acts uniformly, so
that the cell performance is stable. Furthermore, the humidity
adjusting films 30 and 31 of the present invention show a specified
mean thickness and a specified moisture permeability. If such
humidity adjusting films 30 and 31 are used, both dry-up and
flooding can be prevented over a broad range of operating
conditions.
[0030] More concretely, the mean thickness of the abovementioned
humidity adjusting films 30 and 31 is 100 .mu.m or less (preferably
80 .mu.m or less, even more preferably 70 .mu.m or less, and most
preferably 60 .mu.m or less). If the humidity adjusting films 30
and 31 are too thick, it becomes difficult to prevent both dry-up
and flooding no matter how the moisture permeability of these films
is adjusted. On the other hand, if the humidity adjusting films 30
and 31 are made thin and controlled to a thickness within the
abovementioned range, then both dry-up and flooding can be
prevented over a broad range of operating conditions by further
appropriately adjusting the moisture permeability of the films.
Furthermore, the mean thickness of the humidity adjusting films 30
and 31 is 5 .mu.m or greater (preferably 10 .mu.m or greater, even
more preferably 15 .mu.m or greater, and most preferably 20 .mu.m
or greater). If the films 30 and 31 are too thin, the reaction gas
tends to pass through so that the electromotive force (OCV) drops;
moreover, the nap or indentations and projections on the surfaces
of the carbon fiber collector layers 40 and 41 penetrate through
the films 30 and 31 so that there is a danger of damage to the
catalyst electrode layers 20 and 21. Furthermore, the
abovementioned mean thickness is determined by dividing the
cross-sectional areas of the films 30 and 31 by the bottom-side
lengths of these films.
[0031] The moisture permeability of the humidity adjusting films 30
and 31 is 1200 g/m.sup.2 hr or greater (preferably 1500 g/m.sup.2
hr or greater, and even more preferably 1700 g/m.sup.2 hr or
greater), but no greater than 4000 g/m.sup.2 hr (preferably 3800
g/m.sup.2 hr or less, and even more preferably 3700 g/m.sup.2 hr or
less). Both dry-up and flooding can be prevented over a broad range
of operating conditions only be controlling the film thickness and
moisture permeability to the above-mentioned ranges. If the
moisture permeability exceeds the upper limit of the abovementioned
range of numerical values, dry-up may occur depending on the
operating conditions; on the other hand, if the moisture
permeability is smaller than the lower limit of the abovementioned
range of numerical values, flooding may occur depending on the
operating conditions. Furthermore, the moisture permeability is the
value determined by the method stipulated in Japanese Industrial
Standard (JIS) L 1099 (B-1).
[0032] The mean thickness and moisture permeability of the
abovementioned humidity adjusting films 30 and 31 can be adjusted
by appropriately combining the calendering and drawing described
later.
[0033] Furthermore, it is necessary that the humidity adjusting
films 30 and 31 be electrically connected to the catalyst electrode
layers 20 and 21 and carbon fiber collector layers (gas diffusion
layers) 40 and 41. For example, the through-type electrical
resistance of the humidity adjusting films 30 and 31 is 30
m.OMEGA.cm.sup.2 or less, preferably 20 m.OMEGA.cm.sup.2 or less,
and even more preferably 15 m.OMEGA.cm.sup.2 or less. A smaller
through-type electrical resistance is more desirable, and there are
no particular restrictions on the lower limit of this resistance;
ordinarily, however, this lower limit is approximately 1
m.OMEGA.cm.sup.2 (e.g., the value used is approximately 3
m.OMEGA.cm.sup.2). Furthermore, the abovementioned through-type
electrical resistance is the value determined by the four-terminal
method (1 kHz alternating current, pressure between terminals: 981
kPa, room temperature).
[0034] The above mentioned humidity adjusting films 30 and 31 are
constructed from a conductive carbonaceous powder and a
polytetrafluoroethylene (PTFE), and show conductivity, air
permeability and hydrophobic properties overall. The abovementioned
conductive carbonaceous powder is used to obtain the conductivity,
air permeability and hydrophobic properties of the humidity
adjusting films 30 and 31; for example, various types of carbon
black such as furnace black, lamp black, thermal black, acetylene
black or the like, or graphite or the like, may be used. These
substances may be used singly, or in mixtures consisting of two or
more substances. A desirable conductive carbonaceous powder is
acetylene black or a mixture containing acetylene black. Acetylene
black and mixtures containing acetylene black are superior in terms
of conductivity, water-repellent properties and chemical
stability.
[0035] Furthermore, the abovementioned PTFE is used to bind the
conductive carbonaceous powder so as to form a film. This material
is also desirable in that the material can be used to cover the
surface of the conductive carbonaceous powder so as to endow this
powder with water-repellent properties.
[0036] For example, the amount of PTFE used is approximately 5 mass
% or greater (preferably 7 mass % or greater, and even more
preferably 10 mass % or greater), but no more than 60 mass %
(preferably 50 mass % or less, and even more preferably 45 mass %
or less), relative to the total amount of the conductive
carbonaceous powder and polytetrafluoroethylene.
[0037] Furthermore, in addition to the abovementioned PTFE, the
humidity adjusting films 30 and 31 may also contain other
fluororesins if necessary. Examples of such fluororesins include
copolymers of tetrafluoroethylene (copolymers with monomers that
contain fluorine atoms such as hexafluoropropylene or the like, or
monomers that do not contain fluorine atoms such as ethylene or the
like), polyvinylidene fluoride resins, polychlorotrifluoroethylene
resins and the like.
[0038] The humidity adjusting films 30 and 31 of the present
invention can be manufactured by forming into a film a mixture
(kneaded mixture, slurry or the like) obtained by uniformly mixing
the [abovementioned] conductive carbonaceous powder, PTFE and (if
necessary) other fluororesins.
[0039] There are no particular restrictions on the details of the
mixing method or film forming method used; these methods can be
worked with appropriate changes being made by a person skilled in
the art. For instance, the following may be cited as an example of
the manufacturing method used. For example, the abovementioned
mixture (kneaded mixture, slurry or the like) can be prepared by a
universally known method. For example, a kneaded mixture can be
prepared by a dry method or a wet method, and a slurry can be
prepared using a wet method. A dry method is a method in which a
fine conductive carbonaceous powder and a fine powder of a PTFE are
mixed. Specifically, in such a dry method, the abovementioned fine
powders are placed in an appropriate mixer (e.g., a V blender), and
are agitated and mixed; furthermore, an appropriate working
assistant (e.g., mineral spirits) is added and absorbed by the
abovementioned mixture, so that a kneaded mixture is prepared.
Furthermore, the fine conductive carbonaceous powder can be
obtained by pulverizing a conductive carbonaceous powder using a
universally known pulverizer (e.g., ball mill, pin mill,
homogenizer or the like), and it is simple to use a commercially
marketed fine powder as the finely powdered PTFE. Furthermore, in
the working assistant absorption process, it is recommended that
the system be heated following the addition of the working
assistant to the mixture (e.g., to a temperature of approximately
40 to 60.degree. C., especially approximately 50.degree. C.).
[0040] On the other hand, a wet method is a method in which the
conductive carbonaceous powder and PTFE are mixed in water.
Specifically, in such a wet method, a slurry (ink) can be prepared
by mixing in water (in the presence of a surfactant) the raw
materials (conductive carbonaceous powder, PTFE) that have been
finely divided to such a degree that dispersion is possible.
Furthermore, if a mechanical shear force is applied to the slurry
(ink) or a precipitating agent (alcohol or the like) is added
during the abovementioned mixing, the conductive carbonaceous
powder and PTFE will co-precipitate. The resulting co-precipitate
is recovered by filtration and dried; then, a kneaded mixture can
be prepared by absorbing an appropriate working assistant in the
dried product in the same manner as in the abovementioned dry
method. Furthermore, although a fine conductive carbonaceous powder
can be prepared in the same manner as in the abovementioned dry
method, it is simpler to add the powder to water together with a
surfactant, and then disperse the powder in liquid while
pulverizing the powder using a pulverizing means for use in liquid
(e.g., a homogenizer or the like). Furthermore, it is also simple
to use a commercially marketed aqueous PTFE dispersion as the
PTFE.
[0041] A PTFE paste extrusion method can be used to form the
mixture into a film. Specifically, various universally known
methods such as a method in which the mixture is pelletized by
means of preliminary molding, and the pellets are extrusion-molded
from a die or the like and dried (extrusion molding method), a
method in which such pellets are extruded into the form of a cord
by means of an extruder, and this cord-form substance is rolled
between two rolls and dried (bead rolling method), or the like can
be utilized.
[0042] The thickness and moisture permeability of the film can be
adjusted by appropriately devising the abovementioned film
formation process. For example, in cases where the primary molded
film is thick in the extrusion molding method or bead calendering
method, calendering with rolls can be repeated until the film
reaches a specified thickness. Furthermore, depending on the
conditions of manufacture, there may be cases in which the density
becomes excessively high so that the moisture permeability drops.
In such cases, however, the moisture permeability can be increased
by calendering. Thus, the film thickness and moisture permeability
can be adjusted by appropriately combining calendering and drawing.
On the other hand, in the case of a coating method, coating and
drying can be repeated until the film reaches a specified
thickness, and calendering and drawing may be appropriately used in
order to achieve a further adjustment of the thickness and moisture
permeability. Furthermore, the electrical resistance in the
direction of thickness of the film can also be adjusted by
calendering and drawing, as can the air permeability.
[0043] Furthermore, in the drying process of the extrusion molding
method or bead calendering method, it is recommended that the
system be heated to a temperature that allows the removal of the
working assistant (mineral spirits or the like) by volatilization
(e.g., approximately 150 to 300.degree. C., and especially
approximately 200.degree. C.). Furthermore, in the drying process
of the coating method, it is recommended that the system be heated
to a temperature that allows the removal of water by volatilization
(e.g., approximately 100 to 300.degree. C., and especially
approximately 120.degree. C.).
[0044] Furthermore, in the drying process, it is also recommended
that organic impurities (e.g., surfactants used in the wet method)
be carbonized and thus rendered harmless. If surfactants remain,
the moisture permeability of the humidity adjusting film is
conspicuously increased; however, the moisture permeability can be
lowered to an appropriate level by carbonizing such surfactants.
For example, the carbonization temperature is approximately 300 to
400.degree. C. (and especially approximately 350.degree. C.).
Furthermore, the method used to remove organic impurities is not
limited to the abovementioned carbonization treatment; various
methods may be appropriately used in accordance with the types of
impurities involved. For example, depending on the type of
surfactant used, it may be possible to remove the surfactant by
volatilization by heating the system to a temperature of
250.degree. C. or greater; moreover, removal by extraction using a
solvent (e.g., an alcohol or the like) is also possible.
Single Cell
[0045] As is shown in FIG. 2, the abovementioned humidity adjusting
films 30 and 31 of the present invention are used to form a single
cell by being combined with other single cell constituent layers
(polymer electrolyte membrane 10, anode side catalyst electrode
layer 20, cathode side catalyst electrode layer 21, anode side
carbon fiber collector layer 40, cathode side carbon fiber
collector layer 41, separators 60 and 61, and the like).
Furthermore, both of the humidity adjusting films 30 and 31 may be
used as shown in FIG. 2; however, it would also be possible to use
only one of these humidity adjusting films (especially the cathode
side humidity adjusting film 31).
[0046] FIG. 3 is schematic perspective view showing an example in
which a humidity adjusting film 31 of the present invention is used
only on the cathode side. In this example shown in FIG. 3, the
humidity adjusting film 31 of the present invention is laminated on
the inside of the cathode side carbon fiber collector layer 41; a
conventional universally known film is used as the anode side
carbon fiber collector layer 40, without any humidity adjusting
film being laminated. Even in the case of such a single cell, since
a humidity adjusting film 31 is sandwiched between the anode side
catalyst electrode layer 21 and anode side carbon fiber collector
layer 41, both dry-up and flooding can be prevented over a broad
range of operating conditions.
[0047] Furthermore, conventional universally known layers can be
used as the other respective single cell constituent layers
described above; however, the following layers are recommended for
use.
[0048] 1) Polymer Electrolyte Membrane 10
[0049] A perfluoro type electrolyte, hydrocarbon type electrolyte
or the like is preferable for use in the polymer electrolyte
membrane 10; in particular, a perfluoro type electrolyte membrane
is especially desirable. A sulfonic acid type electrolyte membrane
(e.g., Nafion (registered trademark, manufactured by du Pont Co.),
GORE-SELECT (registered trademark, manufactured by Japan Gore-Tex
Inc.) or the like) is preferable, and a perfluorosulfonic acid
resin type electrolyte membrane reinforced by a drawn porous
polytetrafluoroethylene (GORE-SELECT (registered trademark,
manufactured by Japan Gore-Tex Inc.) or the like) is especially
desirable.
[0050] For example, it is recommended that the EW (equivalent
weight) of the polymer electrolyte membrane 10 be approximately 700
or greater (preferably 900 or greater), but no more than 1500
(preferably 1300 or less). Furthermore, for example, it is
desirable that the thickness of the polymer electrolyte membrane be
approximately 10 .mu.m or greater (preferably 15 .mu.m or greater),
but no greater than 100 .mu.m (preferably 60 .mu.m or less)
[0051] 2) Catalyst Electrode Layers 20 and 21
[0052] Conventional universally known layers can be used as the
catalyst electrode layers. For example, layers manufactured from a
paste-form ink obtained by uniformly mixing fine particles of
conductive carbon (mean particle size: approximately 20 to 100 nm)
such as carbon black or the like which has fine particles of
platinum or an alloy of platinum with some other metal (e.g., Ru,
Rh, Mo, Cr, and Fe) (mean particle size 10 nm or less) supported on
the surface, and a liquid containing a perfluorosulfonic acid
resin, in an appropriate solvent (e.g., an alcohol), can be
used.
[0053] It is desirable that the amount of platinum (calculated as
metallic platinum) in the anode side catalyst electrode layer 20
(fuel pole) be approximately 0.1 to 0.5 mg/cm.sup.2, and that the
amount of platinum (calculated as metallic platinum) in the cathode
side catalyst electrode layer 21 (air pole) be approximately 0.3 to
0.8 mg/cm.sup.2.
[0054] For example, the thickness of the catalyst electrode layer
is approximately 5 to 30 .mu.m.
[0055] 3) Carbon Fiber Collector Layers 40 and 41
[0056] The carbon fiber collector layers 40 and 41 must be at least
gas-permeable (air-permeable) and conductive. Woven fabrics,
nonwoven fabrics (felts or the like obtained by entangling carbon
fibers), papers (carbon papers) and the like constructed from
carbon materials are commonly used as such carbon fiber collector
layers 40 and 41. In cases where a comprehensive improvement in the
conductivity, air permeability, water permeability, corrosion
resistance and the like of the carbon fiber collector layers 40 and
41 is to be obtained, a sheet (woven fabric, paper or the like)
that is graphitized by subjecting a sheet-form substance obtained
by the fiber manufacture or papermaking of carbon fibers using
petroleum pitch, phenol, cellulose, acrylonitrile fibers or the
like as a raw material to a heat treatment at a high temperature
(e.g., 1500.degree. C. or greater, preferably 2000.degree. C. or
greater) in an inert gas atmosphere or the like, is ideal for use
as the carbon fiber collector layers 40 and 41. In the case of
nonwoven fabrics (felts or the like), the fiber end surfaces
protrude in the direction thickness, so that when such layers are
laminated with other layers, or when the single cells are stacked,
the polymer electrolyte membrane 10 or catalyst electrode layers 20
and 21 tend to be scratched by these fiber end surfaces. In the
case of woven fabrics or papers on the other hand, the fiber end
surfaces do not protrude, so that damage to the polymer electrolyte
membrane 10 or catalyst electrode layers 20 and 21 can be prevented
to a great extent. Furthermore, woven fabrics or papers consisting
of graphite fibers obtained from an acrylonitrile raw material are
superior in terms of mechanical strength, and are therefore
especially desirable.
[0057] If necessary, furthermore, the carbon fiber collector layers
40 and 41 may be subjected to a water-repellent treatment by means
of a fluororesin. This water-repellent treatment refers to a
treatment in which the carbon fiber collector layers 40 and 41 are
immersed in a liquid containing a fluororesin, and are then dried.
Furthermore, the abovementioned immersion and drying may be
repeated until the desired amount of fluororesin is caused to
adhere [to these layers]. An aqueous dispersion of a fluororesin
using a surfactant can be used as the abovementioned liquid
containing a fluororesin, and the abovementioned commercially
marketed aqueous PTFE dispersion is also one desirable example of
such a liquid containing a fluororesin. If water-repellent
properties are to be securely imparted to the carbon fiber
collector layers 40 and 41, it is recommended that the amount of
fluororesin in the carbon fiber collector layers 40 and 41 be set
at (for example) 0.5 mass % or greater, preferably 5 mass % or
greater, and even more preferably 10 mass % or greater. On the
other hand, if the amount of fluororesin is excessive, the ability
to discharge water is reduced, so that flooding tends to occur.
Accordingly, for example, it is recommended that the amount of
fluororesin contained in the carbon fiber collector layers 40 and
41 be set at approximately 65 mass % or less, preferably 50 mass %
or less, and even more preferably 30 mass % or less.
[0058] There are no particular restrictions on the drying
temperature used for the carbon fiber collector layers 40 and 41
immersed in the abovementioned liquid containing a fluororesin; for
example, this temperature may be approximately 150.degree. C. or
less. Furthermore, it is desirable that any surfactant contained in
the abovementioned liquid containing a fluororesin be appropriately
removed. A method similar to the surfactant removal treatment
method used in the manufacture of the humidity adjusting film
(especially removal by volatilization or a carbonization treatment)
can be used as this removal treatment method. Furthermore, a
carbonization treatment not only renders surfactants harmless, but
also has the effect of fixing the fluororesin in the carbon fiber
collector layers 40 and 41.
[0059] For example, the thickness of the carbon fiber collector
layers 40 and 41 is 100 to 500 .mu.m.
Composite Film
[0060] Furthermore, the humidity adjusting films 30 and 31 of the
present invention may be used without modification in single cells,
or may be used in single cells after being formed into composite
films by laminating and integrating these humidity adjusting films
30 and 31 beforehand with other functional layers (polymer
electrolyte membrane 10, catalyst electrode layers 20 and 21,
carbon fiber collector layers (gas diffusion layers) 40 and 41 or
the like). For instance, the following may be cited as examples of
desirable composite films.
[0061] 1) Humidity adjusting films 100 and 101 equipped with an
electrode function in which humidity adjusting films 30 and 31 and
catalyst electrode layers 20 and 21 are laminated and integrated
(see FIGS. 2 and 3).
[0062] 2) Laminated type gas diffusion layers 110 and 111 in which
humidity adjusting films 30 and 31 and carbon fiber collector
layers (gas diffusion layers) 40 and 41 are laminated and
integrated (see FIGS. 2 and 3).
[0063] 3) Gas diffusion layers 120 and 121 equipped with an
electrode function in which carbon fiber collector layers 40 and 41
are laminated with humidity adjusting films 30 and 31 on one
surface of these humidity adjusting films 30 and 31, and catalyst
electrode layers 20 and 21 are laminated with these humidity
adjusting film 30 and 31 on the other surface.
[0064] 4) A fuel cell membrane electrode assembly 140 in which a
membrane electrode assembly 130 that is integrated by sandwiching a
polymer electrolyte membrane 10 between a pair of catalyst
electrode layers 20 and 21 from both sides is further sandwiched
from both sides by a pair of humidity adjusting films 30 and 31,
and is thus integrated (see FIG. 2).
[0065] 5) A gas diffusion layer integrated type membrane electrode
assembly 150 in which carbon fiber collector layers (gas diffusion
layers) 40 and 41 are laminated and integrated on both outer sides
of the abovementioned membrane electrode assembly 140 (see FIG.
2).
[0066] 6) A membrane electrode assembly 141 in which a humidity
adjusting film 31 is laminated and integrated on the cathode side
of a membrane electrode assembly 130 formed by sandwiching a
polymer electrolyte membrane 10 from both sides between an anode
catalyst electrode layer 20 and a cathode catalyst electrode layer
21 so that these layers are integrated (see FIG. 3).
[0067] 7) A gas diffusion layer integrated type assembly 151 in
which carbon fiber collector layers (gas diffusion layers) 40 and
41 are laminated and integrated on both outer sides of the
abovementioned gas diffusion layer integrated type electrode
assembly 141 (see FIG. 3).
[0068] There are no particular restrictions on the means used to
laminate and integrate the respective layers; conventional
universally known lamination means (e.g., lamination using an
adhesive agent, lamination by heating and pressing or the like) may
be appropriately used; however, if gas permeability is taken into
account, it is desirable to perform a heating and pressing
treatment. Especially in cases where humidity adjusting films 30
and 31 and carbon fiber collector layers 40 and 41 that have been
subjected to a water-repellent treatment are laminated, a drop in
the water-repellent properties can also be prevented by performing
a heating and pressing treatment. Furthermore, in cases where the
humidity adjusting film 30 and 31 and carbon fiber collector layers
40 and 41 are laminated, it is most highly recommended to perform
pressing while heating the films to a temperature of approximately
300 to 400.degree. C. (especially about 350.degree. C.). In cases
where the humidity adjusting films 30 and 31 are manufactured by a
wet method, some surfactant remains, and some surfactant also
remains in cases where the carbon fiber collector layers 40 and 41
are subjected to a water-repellent treatment. If pressing is
performed while heating the films to a temperature of approximately
300 to 400.degree. C. (and especially about 350.degree. C.), the
carbonization of surfactants and the lamination and integration of
the humidity adjusting films 30 and 31 and carbon fiber collector
layers 40 and 41 can be accomplished by performing a single
treatment, which is convenient.
[0069] Furthermore, an assembly obtained by directly coating a
polymer electrolyte membrane 10 with the abovementioned paste-form
ink (used to form the catalyst electrode layers 20 and 21) using a
printing apparatus such as a doctor blade, bar coater or the like,
or an assembly obtained by coating the surface of a smooth film
with good mold release characteristics (consisting of a
polytetrafluoroethylene, polypropylene or the like) beforehand with
the abovementioned paste-form ink using a printing apparatus such
as a doctor blade, bar coater or the like, and drying this coating,
and then transferring the coating layer to a polymer electrolyte
membrane 10 using a hot press (decal method) or the like, may be
used as the abovementioned membrane electrode assembly 130.
Furthermore, "PRIMEA" (registered trademark) which can be obtained
from Japan Gore-Tex Inc. may also be used as the membrane electrode
assembly (MEA) 130.
[0070] The humidity adjusting films 30 and 31 of the present
invention can prevent both dry-up and flooding over a broad range
of operating conditions. Accordingly, these humidity adjusting
films can be used in various types of fuel cells, both in mobile
bodies (automobiles or the like) and for household use. Especially
in the case of mobile bodies (automobiles or the like) there are
frequent fluctuations in the load of the fuel cell during starting,
driving and stopping, so that desirable operating conditions of the
fuel cell may vary according to the operating mode; however, the
humidity adjusting films 30 and 31 of the present invention are
effective in preventing both dry-up and flooding under any
operating conditions.
[0071] Below, the present invention will be described more
concretely in terms of working examples. However, the present
invention is not limited in any way by the following working
examples; the present invention can of course be worked with
appropriate alterations being made within limits that agree with
the main point of the invention as described above and hereafter.
All such alterations are included in the technical scope of the
present invention.
Working Example 1
[0072] Acetylene black (a conductive carbonaceous powder) was
placed gently in water so as to prevent scattering, and water was
absorbed by the acetylene black while these ingredients were mixed
using an agitator. Next, the acetylene black was agitated and
dispersed using a homogenizer, thus producing an aqueous dispersion
of acetylene black.
[0073] A specified amount of an aqueous dispersion of PTFE
(commercial name: D1-E, manufactured by Daikin Industries Ltd.) was
added to this aqueous dispersion of acetylene black, and these
ingredients were gently stirred by means of an agitator, thus
producing a uniform mixed dispersion. Next, the rotation of the
agitator was increased, thus causing the co-precipitation of PTFE
and acetylene black. The co-precipitate was filtered and collected,
and was thinly spread in a stainless steel vat. This co-precipitate
was then dried for one day and night at 120.degree. C., thus
producing a mixed powder of acetylene black (conductive
carbonaceous powder) and PTFE.
[0074] Mineral spirits (manufactured by Idemitsu Kosan Co., Ltd.,
commercial name: IP Solvent 1016) were added to this mixed powder
as a working assistant, the powder was pelletized using a
preliminary molding machine, and the pellets were extruded into the
form of a tape using an extruder; a film was then formed by further
calendering this tape using a double roll. Furthermore, calendering
was performed a multiple number of times using this double roll, so
that the thickness and density of the film were adjusted. The
rolled product was dried for 8 hours in a drier at 200.degree. C.
so that the mineral spirits were removed, and a humidity adjusting
film was then obtained by performing a heat treatment for 5 minutes
at 350.degree. C.
[0075] Details concerning the humidity adjusting film of Working
Example 1 are as shown in Table 1. The mass ratio of acetylene
black to PTFE was 60/40, the mean thickness was 25 .mu.m, the
moisture permeability was 3300 g/m.sup.2 hr, and the through-type
resistance was 8.2 m.OMEGA.cm.sup.2.
[0076] Furthermore, the abovementioned mass ratio, thickness,
moisture permeability and through-type resistance were values that
were determined as shown below.
Mass Ratio
[0077] The mass ratio was calculated on the basis of the amount of
acetylene black used and the solid content of the PTFE aqueous
dispersion.
Mean Thickness
[0078] The cross-sectional area of the humidity adjusting film was
measured using an optical microscope, and the mean thickness was
determined by dividing this cross-sectional area by the length of
the lower side.
Moisture Permeability
[0079] The moisture permeability was determined by the method
stipulated in JIS L 1099 (B-1).
Through-Type Resistance
[0080] The humidity adjusting film was clamped between a pair of
gold-plated metal blocks (area 2 cm.sup.2) (pressure: 981 kPa (10
kgf/cm.sup.2), four-terminal method), and the resistance value in a
case where a 1 kHz alternating current was caused to flow (current:
100 mA) was measured using a m.OMEGA. meter (manufactured by Adex
Co., commercial name: Digital Battery m.OMEGA. Meter (Model
AX-126B). The through-type resistance was then determined using the
following equation: Through-type resistance (m
.OMEGA.cm.sup.2)=measured resistance value (m .OMEGA.).times.2
(cm.sup.2)
[0081] Working Examples 2 Through 5 and Comparative Examples 1
Through 5
[0082] Humidity adjusting films were obtained in the same manner as
described in Working Example 1, except for the fact that the type
of conductive carbonaceous powder, ratio of conductive carbonaceous
powder to PTFE, film thickness, moisture permeability, through-type
resistance and the like were altered. Details of these alterations
are as shown in Tables 1 and 2. Furthermore, in Tables 1 and 2,
"Vulcan XC72-R" (commercial name) manufactured by Cabot Co. was
used as the "furnace black".
Example of Manufacture
[0083] The single cell shown in FIG. 2 was manufactured using the
humidity adjusting films of the working examples and comparative
examples obtained as described above. Furthermore, these single
cells were manufactured as follows.
Laminated type Gas Diffusion Layers 110 and 111
[0084] A carbon paper (commercial name T-GP-H060, manufactured by
Toray) was immersed in a treatment solution adjusted to a PTFE
concentration of 10% by diluting an aqueous dispersion of PTFE
(commercial name: D1-E, manufactured by Daikin Industries Ltd.)
with water, and was then pulled out of this solution. After the
excess treatment solution on the surface of the carbon paper was
wiped away, the carbon paper was dried for 1 hour at 150.degree.
C., and was then subjected to a heat treatment for 2 hours at
350.degree. C., thus producing carbon fiber collector layers 40 and
41 (PTFE content: 18 mass %) subjected to a water-repellent
treatment.
[0085] The humidity adjusting films 30 and 31 obtained in Working
Examples 1 through 5 or Comparative Examples 1 through 5 were
superimposed with the abovementioned carbon fiber collector layer
40 so that no wrinkles were generated, and were subjected to a
pressing treatment by means of a double roll heated to 300.degree.
C., thus producing laminated type gas diffusion layers 110 and
111.
Membrane Electrode Assembly 130
[0086] "PRIMEA" (registered trademark, manufactured by Japan
Gore-Tex Inc.) obtained by bonding catalyst layers 20 and 21
containing 0.3 mg/cm.sup.2 platinum to both surfaces of
"GORE-SELECT" (registered trademark), commercial name of a product
manufactured by Japan Gore-Tex Inc. with a thickness of 30 .mu.m
(polymer electrolyte membrane 10), was used as the membrane
electrode assembly 130.
Single Cell
[0087] The abovementioned laminated type gas diffusion layers 110
and 111 were disposed on both sides of the abovementioned membrane
electrode assembly 130, and a gasket (not shown in the figures) was
superimposed on the outer circumferential part of the polymer
electrolyte membrane 10; then, this assembly was further clamped
from both sides by a pair of graphite separators 60 and 61 in which
gas flow passages were formed. Next, this assembly was clamped by
two stainless steel end plates (not shown in the figures) equipped
with collector plates, thus producing a single cell.
[0088] Hydrogen gas and air were supplied to the single cell thus
obtained, and this cell was operated under various conditions A
through D while the gas and air heating temperatures and air
utilization rate (air flow rate) were varied as shown in Tables 1
and 2. The voltage across the terminals was measured at the
open-circuit voltage (OCV) and with the current density set at 1.4
A/cm.sup.2. Furthermore, the cell temperature (80.degree. C.) and
the fuel hydrogen utilization rate (80%) were constant.
[0089] The results obtained are shown in Tables 1 and 2.
TABLE-US-00001 TABLE 1 Humidity adjusting film Ratio of Mean
Moisture Internal Carbon carbon thickness permeability resistance
powder PTFE powder/PTFE (.mu.m) (g/m.sup.2hr) (m.OMEGA.cm.sup.2)
Working Acetylene D1-E 60/40 25 3300 8.2 Example 1 black Working
Acetylene D1-E 75/25 50 2200 12.5 Example 2 black Working Acetylene
D1-E 80/20 60 3600 8.8 Example 3 black Working Furnace D1-E 90/10
25 1900 5.8 Example 4 black Working Furnace D1-E 80/20 60 3500 5.9
Example 5 black Operating Operating conditions results Hydrogen
Terminal humidification Air humidification voltage at (relative
humidity (relative humidity Air utilization OCV 1.4 A/cm.sup.2 No.
%) %) rate (%) (mV) (mV) Working A 100 100 40 927 606 Example 1 B
100 100 70 926 537 C 40 40 40 948 506 D 40 40 70 947 481 Working A
100 100 40 953 472 Example 2 B 100 100 70 951 251 C 40 40 40 979
354 D 40 40 70 980 250 Working A 100 100 40 940 527 Example 3 B 100
100 70 944 293 C 40 40 40 980 372 D 40 40 70 976 256 Working A 100
100 40 942 536 Example 4 B 100 100 70 952 367 C 40 40 40 989 317 D
40 40 70 987 310 Working A 100 100 40 929 527 Example 5 B 100 100
70 930 347 C 40 40 40 949 446 D 40 40 70 947 352
[0090] TABLE-US-00002 TABLE 2 Humidity adjusting film Ratio of Mean
Moisture Internal Carbon carbon thickness permeability resistance
powder PTFE powder/PTFE (.mu.m) (g/m.sup.2hr) (m.OMEGA.cm.sup.2)
Comparative Acetylene D1-E 60/40 3 4000 5.4 Example 1 black
Comparative Acetylene D1-E 70/30 120 2050 27 Example 2 black
Comparative Acetylene D1-E 70/30 60 600 15 Example 3 black
Comparative Furnace D1-E 70/30 60 4100 10.4 Example 4 black
Comparative Furnace D1-E 60/40 110 1050 6.4 Example 5 black
Operating Operating Conditions results Hydrogen Terminal
humidification Air voltage at (relative Air humidification
utilization OCV 1.4 A/cm.sup.2 No. humidity %) (relative humidity
%) rate (%) (mV) (mV) Comparative A 100 100 40 900 528 Example 1 B
100 100 70 901 301 C 40 40 40 913 373 D 40 40 70 915 241
Comparative A 100 100 40 940 466 Example 2 B 100 100 70 936 366 C
40 40 40 958 unmeasurable D 40 40 70 960 unmeasurable Comparative A
100 100 40 950 296 Example 3 B 100 100 70 953 unmeasurable C 40 40
40 962 347 D 40 40 70 960 233 Comparative A 100 100 40 948 512
Example 4 B 100 100 70 947 295 C 40 40 40 984 113 D 40 40 70 980
unmeasurable Comparative A 100 100 40 942 424 Example 5 B 100 100
70 940 unmeasurable C 40 40 40 963 311 D 40 40 70 961
unmeasurable
[0091] Comparative Example 1 showed a drop in OCV possibly because
of a tendency for hydrogen gas to leak due to excessive thinness of
the humidity adjusting film. In Comparative Examples 2 through 5,
the humidity adjusting film was too thick, or the moisture
permeability was inappropriate; accordingly, when power was
generated at a high output (1.4 A/cm.sup.2), flooding or dry-up
occurred under some of the operating conditions A through D, so
that measurement of the voltage across the terminals was
impossible. To describe this more accurately, dry-up occurred under
the operating conditions C and D in Comparative Example 2, flooding
occurred under the operating conditions B in Comparative Example 3,
and dry-up occurred under the operating conditions C and D in
Comparative Example 4. In particular, both flooding (under the
operating conditions B) and dry-up (under the operating conditions
D) occurred in Comparative Example 5.
[0092] On the other hand, in the case of the humidity adjusting
films of Working Examples 1 through 5, since both the film
thickness and moisture permeability were appropriate, it was
possible to generate power over a broad range of operating
conditions without any occurrence of flooding or dry-up, regardless
of whether the humidification was low (40%) or high (100%), and
regardless of whether the gas flow rate was high (air utilization
rate 40%) or low (air utilization rate 70%).
[0093] While particular embodiments of the present invention have
been illustrated and described herein, the present invention should
not be limited to such illustrations and descriptions. It should be
apparent that changes and modifications may be incorporated and
embodied as part of the present invention within the scope of the
following claims.
* * * * *